1. Field of the Invention
The invention is related to light emitting diodes (LEDs), and more particularly, to new structures for producing white, single or multi-color LEDs with high extraction efficiency by recycling guided modes.
2. Description of the Related Art
(Note: This application references a number of different publications and patents as indicated throughout the specification by one or more reference numbers within brackets, e.g., [x]. A list of these different publications and patents ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications and patents is incorporated by reference herein.)
A light emitting diode (LED) is a semiconductor device that emits light when electrically biased in the forward direction. This effect is a form of electroluminescence.
LEDs are typically comprised of a chip of semiconducting material impregnated or doped with impurities to create a structure known as a pn junction. When forward biased, electrons are injected into the pn junction from an n-region of the device and holes are injected into the pn junction from a p-region of the device. The electrons and holes release energy in the form of photons as they recombine. The wavelength of the light, and therefore its color, depends on the bandgap energy of the materials forming the pn junction.
As semiconductor materials have improved, the efficiency of LEDs has also improved, and new wavelength ranges have been used. For example, gallium nitride (GaN) based LEDs are probably the most promising for a variety of applications. GaN provides efficient illumination in the ultraviolet (UV) to amber spectrum, when alloyed with varying concentrates of indium (In), for example.
Unfortunately, most of the light emitted within an LED is lost due to internal reflections at the semiconductor-air interface. Typical semiconductor materials have a high index of refraction, and thus, according to Snell's law, most of the light will remain trapped in the materials, thereby degrading efficiency. However, by choosing a suitable geometry for the LED, a higher extraction efficiency can be achieved, wherein extraction efficiency refers to the ability of the photons generated by a particular system to actually escape the materials as “useful” radiation, i.e., extracted light.
FIG. 1 is a cross-sectional view of a semiconductor LED 100 that illustrates how a portion of the light 102 emitted from a QW 104 traverses an escape cone 106 in order to be extracted from the device 100, while a large fraction of the emitted light 108 is trapped and reflected within the device 100. In this situation, the reflected light 108 is referred to as guided light modes, or guided modes, because the light 108 is confined within the device 100 and guided transversely within the semiconductor materials comprising the device 100.
One method to reduce the effects of the internal reflection is to create light redistribution through random texturing of the surface of the device, which leads to multiple variable-angle incidence at the semiconductor-air interface of the device.
FIG. 2 is a cross-sectional view of a semiconductor LED 200 that illustrates this concept, wherein the top surface 202 of the LED 200 is textured, the bottom surface 204 of the LED 200 comprises a reflector, the air is shown to have a refractive index of n=1, and the semiconductor material of the LED 200 is shown to have a refractive index of n=3.5.
The textured-surface approach has been shown to improve emission efficiency to approximately 9–40%, due to the very high internal efficiency and low internal losses of the device, which allows many reflections or passes for the emitted light before it is extracted from the device. [1,2]
Another method to reduce the percentage of light trapped is to use a micro-cavity LED (MCLED), also known as a resonant cavity LED (RCLED). [3,4] MCLEDs offer opportunities to create solid-state lighting systems with greater efficiencies than existing systems using traditional LEDs. As a result of incorporating an active medium within a resonant cavity, MCLEDs emit a highly compact and directional light beam. The higher extraction efficiency and greater brightness of these devices are their main advantages over conventional LEDs. This higher extraction efficiency is, however, limited to values in the 40% range as the micro-cavity structure also leads to very efficient emission into guided modes. Thus, it would be useful if these guided modes could be extracted.
Beyond simple monochrome LEDs, the generation of high-efficiency, good color-rendering, white LEDs is one of the most important goals the industry is trying to achieve. White light is currently made in one of two ways:                1. By selectively combining the proper combination of red, green and blue (RGB) LEDs. However, this solution is costly and the overall light output of each RGB LED degrades at a different rate, thereby resulting in an eventual color imbalance.        2. By using a phosphor coating, typically yttrium aluminum garnet (YAG), on a surface of a blue LED. The blue LED excites the phosphor, thereby causing it to glow white (Nichia). This is the dominant method of achieving white light output. Alternatively, this method may use a UV-emitting LED in combination with a luminescence conversion LED (LUCOLED).        
FIG. 3A is a cross-sectional view of the structure of a white LED 300 comprised of a gallium indium nitride (GaInN) blue LED die 302, a phosphor-containing epoxy 304 encapsulating the die 302, and bond wires 306 leading from the die 302 to a package 308, which is sealed by a cap 310. FIG. 3B is a cross-sectional view that illustrates the wavelength-converting phosphorescence 312 and blue luminescence 314.
The problems with the structure of FIGS. 3A and 3B are the poor efficiencies of the LED, the optical coupling between the LED and phosphor, the low brightness, and the non-planar fabrication technique.
Another structure that may be used to obtain a white LED is the photon recycling semiconductor LED (PRS-LED), which comprises an epitaxially-grown indium gallium nitride (InGaN) based blue LED bonded to a second wafer containing an aluminum gallium indium phosphide (AlGaInP) active region. FIG. 4 is a cross-sectional view of the structure of a PRS-LED 400, wherein the PRS-LED 400 is comprised of a sapphire substrate 402, a p-GaN layer 404, a primary InGaN active region 406 emitting light 408 in the blue wavelength range, an n-GaN layer 410, and an electrically-inactive AlGaInP photon recycling wafer 412 re-emitting a complementary colored light 414, such as yellow/orange. A p-type contact 416 and n-type contact 418 are placed on the bottom of the PR+S-LED 400.
The first PRS-LED was demonstrated as a hybrid device by Guo and colleagues in 1999. [5] This device emits two discrete wavelengths, and the combined output should be perceived as white light. The PRS-LED can also be designed to emit other colors by the proper combination of emitting species, which is not possible with conventional LEDs. In addition, more recycling layers are possible, giving rise to bi-chromatic and tri-chromatic PRS-LEDs. The drawbacks here are the mediocre optical coupling efficiency between emitters and the poor efficiency of the active blue LED.
Notwithstanding the above, what is needed in the art are new LED structures that provide white, single or multi-color light and increased light extraction efficiency, while retaining a planar structure, so that they are easily manufacturable at low cost. The present invention solves that need.